1. Field of the Invention
This invention relates to a method for heating a permeable, thermoformable material using heated fluid to convectively heat the material, wherein flow of the fluid is regulated such that relatively cooler portions of the fluid travel more quickly than hotter portions of the fluid.
2. Background Art
A prior method of heating a thermoformable material, including thermoset and thermoplastic materials, involves contact heating. The contact heating method includes placing the material in direct contact with a heat source, such as thermally regulated upper and lower platens. Heat is then transferred principally by conduction from the outer surface to the inner core of the material. Because such materials are typically poor conductors of heat, however, this method requires a significant amount of time to sufficiently heat the materials.
Another method of heating a thermoformable material involves radiant heating. This method involves placing the material near a radiant heat source, such as electric coils or ceramic heaters. The outer portions of the material, however, tend to selectively absorb the radiant energy, and core heating is again accomplished primarily by conduction. Consequently, this method also requires a significant amount of time to sufficiently heat the material.
U.S. Pat. No. 6,036,896, which is assigned to the assignee of the present invention, discloses a method of heating a permeable, thermoformable material using convective heating. The method involves supplying heated fluid to a fluid distribution system and homogenizing t he fluid such that the fluid has a substantially uniform velocity and temperature. The method further involves drawing the fluid through the material and passing the fluid through a flow regulating device so as to maintain the homogeneity of the fluid as the fluid exits the material.
Under the invention, a method for preheating a permeable, thermoformable material having first and second sides includes positioning the material proximate an outlet of a fluid distribution system such that a first section of the material is disposed further away from the outlet than a second section of the material; supplying heated fluid through the outlet; regulating flow of the fluid; introducing the fluid onto the first side of the material; and developing a suction on the second side of the material sufficient to draw the fluid through the material thereby convectively heating the material; wherein the flow of the fluid is regulated such that a first portion of the fluid having a first velocity and a first temperature passes through the first section of the material, and a second portion of the fluid having a second velocity less than the first velocity and a second temperature greater than the first temperature passes through the second section of the material, such that substantially uniform energy flux is transferred from the first and second portions of the fluid to the first and second sections, respectively, of the material.
Advantageously, substantially uniform energy flux may be transferred from the first and second portions of fluid to the first and second sections, respectively, of the material during initial or transient flow conditions, as well as during later steady state flow conditions. As a result, the material may be efficiently and effectively heated without necessarily requiring steady state flow conditions to be reached. Thus, heating cycle times can be reduced.
Further under the invention, a method for preheating a permeable, thermoformable material having first and second sides includes supplying heated fluid to a fluid distribution system; regulating flow of the fluid such that fluid having a first temperature flows at a first velocity, and fluid having a second temperature less than the first temperature flows at a second velocity greater than the first velocity; introducing the fluid onto the first side of the material; and developing a suction on the second side of the material sufficient to draw the fluid through the material thereby convectively heating the material; wherein the flow of the fluid is regulated so as to transfer substantially uniform energy flux to the material.
More specifically, a method for preheating a permeable, thermoformable material having first and second sides includes positioning the material in a housing having a fluid inlet such that a first section of the material is disposed further away from the inlet than a second section of the material; supplying heated fluid to the inlet; passing the fluid through a first flow regulating device having first and second openings such that a first portion of fluid passes through the first opening and a second portion of fluid passes through the second opening, wherein the first opening is disposed further away from the outlet than the second opening, and wherein the first opening is larger than the second opening, such that after passing through the first flow regulating device the first portion of fluid has an average first temperature and an average first velocity, and the second portion of fluid has an average second temperature greater than the average first temperature and an average second velocity less than the average first velocity; passing the fluid through a second flow regulating device disposed downstream of the first flow regulating device so as to homogenize each of the first and second portions of the fluid; introducing the fluid onto the first side of the material; developing a suction on the second side of the material sufficient to draw the fluid through the material such that the first portion of the fluid passes through the first section of the material and the second portion of the fluid passes through the second section of the material, thereby transferring substantially uniform energy flux from the first and second portions of the fluid to the first and second sections, respectively, of the material; and passing the fluid through a third flow regulating device disposed downstream of the material so as to maintain the homogeneity of each of the first and second portions of the fluid.
These and other objects, features and advantages of the invention are readily apparent from the following detailed description of the preferred embodiments for carrying out the invention, when taken in conjunction with the accompanying drawings.